If we register a building encumbrance and the neighbor sells, we may have to remove it again or renegotiate it with the buyer.
That is exactly not possible, since the building encumbrance is not a contract between private individuals but an entry in a public register here, the building encumbrance register. The building encumbrance remains in place until deletion is requested and the building authority (unilaterally!) examines whether the building encumbrance is still needed. If it is no longer needed, it will be deleted. Otherwise, it remains.
If that is not possible, it will be dismantled. Meaning the garage slightly lowered, which is fortunately possible because it has a relatively high structure on top of the garage that can be removed with some effort. And of course remove the stairs and order an attached balcony and then install the stairs there.
Simply dismantling down to 3m will not be enough; the roof terrace must also be dismantled. But none of that is dramatic; it can be — as far as can be judged from a distance — resolved cleanly.
What I find somewhat surprising, as does , is that you got approval/acceptance for a 3.4m high garage without a building encumbrance. That may be possible in some cases, but it is definitely somewhat unusual.
However, you can initially contact a publicly appointed surveyor near you without obligation to clarify how to proceed most cleverly without directly raising alarm at the building authority.
Best regards
Dirk Grafe